The following source states the RCS to be 0.5m2 and I don't claim it to be 100% authentic, but it seems closer to the estimate of 1 to 3m2 than this 5m2 claim. Unless I come across a source which specifically states the RCS is more than 3m2, I am more inclined to think it to be anywhere from 0.5 to 3m2.
Deagel : Su-35
Su-35 Specifications
Crew: 1
External Stations: 12
Number of Engines: 2
Service Life Flight Hours: 6,000
Area
Radar Cross Section (RCS): 0.50 square meter (5,000 square cm)
Dimensions
Height: 5.90 meter (19.4 foot)
Length: 21.9 meter (72 foot)
Wingspan: 15.3 meter (50 foot)
G-load
Max Maneuvering Load Factor: 9 g
Performance
Ceiling: 19,000 meter (62,336 foot)
Ferry Range: 4,500 kilometer (2,430 nautical mile)
Max Range: 3,600 kilometer (1,944 nautical mile)
Max Range at Sea Level: 1,580 kilometer
Speed
Cruise Speed: 1.20 mach (398 mps)
Top Speed at High Altitude: 2,500 kph (2.09 mach)
Top Speed at Sea Level: 1,400 kph (1.17 mach)
Time
Service Life: 30 year
Time Between Overhaul: 10 year
Weight
Fuel Load: 11,500 kilogram (25,353 pound)
Max Takeoff Thrust: 29,000 kilogram (63,933 pound)
Max Takeoff Weight: 34,500 kilogram (76,058 pound)
Operational Weight: 25,300 kilogram (55,776 pound)
Payload: 8,000 kilogram (17,637 pound)
Main Gun Caliber: 30 millimeter
Gun Ammunition Load: 150